The long term goal of this proposal is to further our understanding of how circadian clocks are built in eukaryotic cells. Circadian clocks are known to regulate many essential cellular processes in all organisms appropriately examined. In higher plants, processes as diverse as photosynthesis and floral induction are subject to circadian regulation. However, the molecular components of these biological clocks remain unknown. We have chosen Arabidopsis as our model system, and we have recently isolated several mutants that exhibit aberrant circadian rhythmicity. Our proposed approach is to use the genetic tools that exist in Arabidopsis to reveal the major components of the Arabidopsis circadian clock. We will extend a mutant screen that has been performed using rhythmic bioluminescence in transgenic seedlings, generated by monitoring the circadian-regulated expression of a CAB::luciferase construct. We will positionally clone the toc1 mutant, which dramatically shortens circadian period of CAB gene expression, CCR gene expression and leaf movements, all of which occur in distinct cell types. TOC1 is therefore likely to encode a central component of the circadian system in Arabidopsis. We will characterize further several other mutant lines, as well as continue our mutant screen using a novel automated procedure. Furthermore, by crossing our CAB::luc marker into many available floral timing mutants, we will be able to relate circadian function to previously defined pathways for photoperiodic control of flowering. Ultimately, we plan to clone the gene(s) that are involved in circadian timing to provide molecular probes for dissecting clock function in higher plants. Given the ubiquity of circadian-regulated physiology, the identification of common clock components will have an impact on understanding the pacemaker mechanism and malfunctions associated with known features of human well-being.